The present invention relates to a circuit arrangement for the timed control of semiconductor switches.
In particular, the invention relates to a circuit arrangement for the timed control of semiconductor switches, in parallel with each of which there is a freerunning diode, and which are arranged in branches of a bridge in the diagonals of which there is an ohmic-inductive load, in particular a dc motor, two semiconductor switches which lie diagonally opposite each other in the bridge and are closed during a current-application phase, and a freerunning current flowing through the load in the freerunning phase.
In known circuit arrangements of the aforementioned type, the control of, in each case, two semiconductor switches lying diagonally opposite each other in the bridge in the same direction is effected by one control signal so that both semiconductor switches are closed simultaneously at the start of a current-application phase so that during that phase the amount of current in the load increases, and in the following freerunning phase they are placed simultaneously into the non-conductive state. During the freerunning phase the current in the load decreases and flows back via freerunning diodes into a source of voltage, for instance an automobile battery, which feeds the circuit arrangement. The average current which is established in the load, i.e. the consuming device, results in this case from the pulse duty factor of the controlled semiconductor switches, i.e. the ratio of the connect time to the disconnect time, and the time constants of the ohmic-inductive load as well as other load parameters. Depending on which pair of the semiconductor switches that lie diagonally opposite each other in the bridge is controlled in timed fashion while the other pair of semiconductor switches in each case is blocked, the current flows in positive or negative direction in the load. In this known circuit arrangement, therefore, the freerunning diodes conduct the entire current flowing through the load in the freerunning phase (freerunning current). The freerunning current, however, produces a relatively large voltage drop in the freerunning diodes and thus a high loss power.